Sealing system

ABSTRACT

A sealing system is provided which is intended to achieve an improvement in durability. A dust seal is composed of a first dust seal ( 100 ) and a second dust seal ( 200 ) which is arranged at a location nearer to an atmospheric air side than the first dust seal ( 100 ), wherein each of the first dust seal ( 100 ) and the second dust seal ( 200 ) has a seal lip formed so as to be slidable with respect to an outer peripheral surface of the shaft. The first dust seal ( 100 ) has seal lips ( 121, 122 ) made of urethane rubber, and the second dust seal ( 200 ) has only a single seal lip ( 221 ) which is made of a rubber higher in flexibility than urethane rubber, and which extends toward the atmospheric air side.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sealing system provided with a dustseal.

2. Description of the Related Art

For example, in oil hydraulic cylinders for construction machinery, asealing system comprising a plurality of seals is used so as to seal anannular gap between a cylinder (housing) and a piston (shaft). Such asealing system is provided with a main seal for preventing the leakageof fluid to be sealed, such as oil, and a dust seal for preventing dustfrom invading into a hermetic seal area. In general, only one dust sealmade of urethane rubber is arranged at an atmospheric air side.

However, in cases where such a sealing system is used under a verysevere environment, two dust seals are used while being arranged side byside. Here, note that as an example of a sealing system in the case ofbeing used under a very severe environment, there is a case in whichsuch a sealing system is used in an iron ore mining site, and dustincludes, in addition to particulates, frozen substances, substancesadhered to the surface of a piston and mud, etc.

Such a sealing system according to a conventional example will beexplained with reference to FIG. 8 and FIG. 9. FIG. 8 is a schematiccross sectional view showing the state of the use of the sealing systemaccording to the conventional example. FIG. 9 is a plan viewschematically showing a second dust seal in the sealing system accordingto the conventional example.

The illustrated sealing system according to the conventional example isused for the purpose of sealing an annular gap between an innerperipheral surface of a shaft hole formed in a housing 500, and an outerperipheral surface of a shaft 300 inserted through the shaft hole. Inaddition, the sealing system according to this conventional example isprovided with a main seal 600 for preventing fluid to be sealed fromleaking to an atmospheric air side (A), and a dust seal 700 forpreventing dust from invading to the side (O) of the fluid to be sealed.In this conventional example, the dust seal 700 is composed of a firstdust seal 710 that is formed at the side of the main seal 600, and asecond dust seal 720 that is formed at a location nearer to theatmospheric air side than this first dust seal 710.

Here, the housing 500 is composed of a housing main body 510, and amember 520 to be fastened (hereinafter also referred to as ato-be-fastened member) which is constructed so as to be freely fastenedand unfastened with respect to this housing main body 510 by means of abolt or the like.

An annular groove 511 is formed on the inner peripheral surface of theshaft hole in the housing main body 510. The main seal 600 is fitted inthis annular groove 511. Here, note that in this conventional example,the main seal 600 is composed of a U-shaped packing 610 of a U shape incross section, and a back-up ring 620 for protecting an inner peripheralend edge of the U-shaped packing 610 at the atmospheric air side (A).

In addition, an annular concave portion 512 forming a step is formed inan open end portion of the shaft hole in the housing main body 510 atthe atmospheric air side (A). The first dust seal 710 is fitted in thisannular concave portion 512. The first dust seal 710 is composed of ametal ring 711, and a seal body 712 that is integrally formed with thismetal ring 711. This first dust seal 710 is fixed in an axiallypositioned state by fastening the to-be-fastened member 520.

Moreover, an annular concave portion 521 forming a step is formed in anopen end portion of a shaft hole in the to-be-fastened member 520 at theatmospheric air side (A). The second dust seal 720 is fitted in thisannular concave portion 521. Similar to the first dust seal 710, thesecond dust seal 720 is composed of a metal ring 721, and a seal body722 that is integrally formed with this metal ring 721. This second dustseal 720 is fixed in an axially positioned state by fitting a snap ring530 into an annular groove formed on an inner peripheral surface of theshaft hole in the to-be-fastened member 520.

In this manner, in the sealing system according to this conventionalexample, the two dust seals are used while being arranged side by side.Accordingly, it is possible to prevent dust from invading to the side(O) of the fluid to be sealed in a more reliable manner in comparisonwith the case in which only one dust seal is used.

Here, with respect to the first dust seal 710, there is no part cut in acircumferential direction thereof, similar to common dust seals. As aresult, at the time of assembly, an operation of fitting the first dustseal 710 onto the shaft 300 from an end side thereof is needed, or anoperation of inserting the shaft 300 into the first dust seal 710 isneeded.

In contrast to this, with respect to the second dust seal 720, it isexposed to atmospheric air, so in cases where it is used under a verysevere environment, the progress of deterioration thereof is rapid. Forexample, muddy water or the like would adhere and stick to the seal body722, or the muddy water or the like adhered to the seal body 722 wouldbe frozen, and when the shaft 300 is caused to slide relative to theseal body 722 in such a state, wear on a seal lip of the seal body 722would be facilitated.

In this manner, the second dust seal 720 is apt to deteriorate, andhence, is constructed so that it can be replaced with a new one in aneasy manner. More specifically, the second dust seal 720 is of atwo-piece divided structure. That is, it is composed of a circular body,being circular as seen from an upper surface side, which is cut ordivided into two semicircular shaped members, as shown in FIG. 9. Withsuch a structure, each divided member can be detached and attached tothe shaft 300 from an outer peripheral side thereof, as a result ofwhich a replacement of the second dust seal 720 can be carried out in asimple manner only by unfastening the to-be-fastened member 520 from thehousing main body, then removing the second dust seal 720, which hasbeen deteriorated, attaching a new one, and finally fastening theto-be-fastened member 520 again.

However, the second dust seal 720 is prepared as follows. That is, fromthe view point of fabrication, a circular body, which is circular asseen from an upper surface side, is first prepared, and is then dividedby cutting into two pieces. Here, the second dust seal 720 is providedwith the metal ring 721, so in order to cut the metal ring 721, such acutting has to be made by means of a cutting blade or edge having acertain amount of thickness. Here, note that in general, the cutting isdone by the use of a grinder, etc.

As a result, a part of the metal ring 721 and a part of the seal body722 are chipped off by cutting (C1 and C2 in FIG. 9 indicate the partsto be chipped off). Accordingly, even if the two divided members arejoined or combined with each other, a perfect circle can not be formed,and hence, in a state where these members are attached to the housing500, it can not be avoided that a gap occurs between the cut ends of themembers. Specifically, the gap of about 2.5 mm at the maximum occurs.For that reason, the invasion of dust from this gap can not beprevented.

As a measure to cope with this, the applicant of this application hasalready filed an application (Japanese patent application 2007-63531)with respect to a technique which adopts, as a second dust seal, onewhich is made of only urethane rubber, and which is cut only at onelocation in a circumferential direction by means of a thin cutting edgewhich is so thin as to make a chipping allowance unnecessary.

However, in the case of this technique, the second dust seal has a cutportion, so it is particularly difficult to prevent the invasion ofminute or fine dust to a sufficient extent. Once dust enters from thesecond dust seal, wear of a seal lip thereof will be facilitated, andthe invasion of dust will be promoted.

The dust having entered from the second dust seal is prevented fromfurther coming into a main seal side, by means of a first dust seal.However, the dust accumulates in a sealed area between the first dustseal and the second dust seal. The dust thus accumulated in this sealedarea, being in a state sealed by the first dust seal and the second dustseal, can not escape from this sealed area. Accordingly, when the amountof accumulation of the dust exceeds a predetermined amount, a load(pressure due to the accumulated dust) is applied to the first dust sealand the second dust seal. Thus, there is a fear that the first dust sealand the second dust seal may be damaged by this load.

The accumulation of dust is more apt to occur in the case of minute dust(i.e., dust having a particle diameter of 10 μm or less) such asparticulates than in the case of frozen substances, mud and the like. Inaddition, in cases where the second dust seal is composed of arelatively hard material such as urethane rubber, such minute dust canalso enter not only from the cut portion but also from the tip side ofthe seal lip.

In this manner, the dust which has entered from the second dust seal andhas accumulated becomes a cause of damaging the first dust seal and thesecond dust seal, thus resulting in a cause of shortening the lives ofthese dust seals.

Here, note that as related techniques, there are those which aredisclosed in a first through a fourth patent document, as listed below.

Prior Art References Patent Documents

[First Patent Document] Japanese utility model examined publication No.H5-14050

[Second Patent Document] Japanese patent application laid-open No.2001-355739

[Third Patent Document] Japanese utility model application laid-open No.S59-22360

[Fourth Patent Document] Japanese utility model application laid-openNo. H5-83410

SUMMARY OF THE INVENTION

The object of the present invention is to provide a sealing system whichis intended to improve its durability.

In order to solve the above-mentioned problems, the present inventionadopts the following measures.

That is, the sealing system of the present invention resides in asealing system which serves to seal an annular gap between an innerperipheral surface of a shaft hole formed in a housing and an outerperipheral surface of a shaft which is inserted through the shaft holeand carries out a reciprocating movement relative to the housing, thesystem comprising:

a main seal that prevents fluid to be sealed from leaking to anatmospheric air side; and

a dust seal that is arranged at a location nearer to the atmospheric airside than the main seal, for preventing dust from invading to a side ofthe fluid to be sealed;

wherein the dust seal is composed of a first dust seal and a second dustseal which is arranged at a location nearer to the atmospheric air sidethan the first dust seal, and each of the first dust seal and the seconddust seal has a seal lip formed so as to be slidable with respect to theouter peripheral surface of the shaft;

the seal lip of the first dust seal is made of urethane rubber; and

the seal lip of the second dust seal is made of a rubber which is higherin flexibility than the urethane rubber, and is composed of only a seallip that extents toward the atmospheric air side.

Here, the seal lip made of urethane rubber is excellent in the functionof scraping dust adhered to the outer peripheral surface of the shaft.On the other hand, the seal lip made of a rubber (e.g., NBR), which ishigher in flexibility than urethane rubber, is excellent in the functionof suppressing the invasion of fine dust.

In the present invention, as described above, the second dust seal withits seal lip made of a rubber more flexible than urethane rubber isarranged at the atmospheric air side. Accordingly, the invasion of finedust can be suppressed by means of this second dust seal. In addition,the dust which has invaded from the second dust seal and has adhered tothe outer peripheral surface of the shaft is scraped by means of thefirst dust seal having its seal lip made of urethane rubber. Further,because the seal lip of the second dust seal is composed of only a seallip extending toward the atmospheric air side, the dust scraped by thefirst dust seal 100 is discharged to the atmospheric air side from aseal lip tip end side of the second dust seal. Accordingly, it ispossible to prevent the dust from accumulating in a sealed area betweenthe first dust seal and the second dust seal, thus making it possible tosuppress a load from being applied to these dust seals.

As explained above, according to the present invention, an improvementin durability of a sealing system can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a sealing system accordingto an embodiment of the present invention.

FIG. 2 is a schematic cross sectional view of a first dust seal and asecond dust seal according to a first example of the present invention.

FIG. 3 is a schematic cross sectional view of a first dust seal andsecond dust seals according to a second example of the presentinvention.

FIG. 4 is a schematic cross sectional view of first dust seals and asecond dust seal according to a third example of the present invention.

FIG. 5 is a schematic cross sectional view of a first dust seal andsecond dust seals according to a fourth example of the presentinvention.

FIG. 6 is a schematic cross sectional view of first dust seals andsecond dust seals according to a fifth example of the present invention.

FIG. 7 is a schematic cross sectional view of a first dust seal and asecond dust seal according to a sixth example of the present invention.

FIG. 8 is a schematic cross sectional view showing the state of the useof a sealing system according to a conventional example.

FIG. 9 is a plan view schematically showing a second dust seal in thesealing system according to the conventional example.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, best modes for carrying out the present invention will bedescribed in detail by way of example based on a preferred embodimentand examples thereof with reference to the accompanying drawings.However, the dimensions, materials, shapes, relative arrangements and soon of component parts described in the embodiment and examples are notintended to limit the scope of the present invention to these alone inparticular as long as there are no specific statements.

Embodiment

A sealing system according to an embodiment of the present inventionwill be explained with reference to FIG. 1. FIG. 1 is a schematic crosssectional view of a sealing system according to an example of thepresent invention.

<Sealing System>

The illustrated sealing system according to this embodiment is used forthe purpose of sealing an annular gap between an inner peripheralsurface of a shaft hole formed in a housing 500, and an outer peripheralsurface of a shaft 300 inserted through the shaft hole. The sealingsystem is used, as a more specific example, for an oil hydrauliccylinder for construction machinery, etc. In the case of the oilhydraulic cylinder, a cylinder corresponds to the housing 500, and apiston corresponds to the shaft 300, wherein the shaft 300 carries out areciprocating movement in the housing 500.

In addition, the sealing system according to this embodiment is providedwith a buffer ring 400 that serves to buffer the pressure of fluid to besealed (e.g., oil), a main seal 600 that prevents the fluid to be sealedfrom leaking to an atmospheric air side (A), and a first dust seal 100and a second dust seal 200 that prevent dust from invading into orentering the side (O) of the fluid to be sealed (hereinafter alsoreferred to as the to-be-sealed fluid side).

Here, the housing 500 is composed of a housing main body 510, and amember 520 to be fastened (hereinafter also referred to as ato-be-fastened member) which is constructed so as to be freely fastenedand unfastened with respect to this housing main body 510 by means of abolt or the like.

A first annular groove 511 and a second annular groove 514 are formed onan inner peripheral surface of the shaft hole in the housing main body510. In addition, the buffer ring 400 is fitted in the first annulargroove 511, and the main seal 600 is fitted in the second annular groove514. Here, note that in this example, the main seal 600 is composed of aU-shaped packing 610 of a U shape in cross section, and a back-up ring620 for protecting an end edge of the U-shaped packing 610 at theatmospheric air side (A). The U-shaped packing 610 is provided with aseal lip which slides with respect to the shaft 300.

In addition, an annular concave portion 513 forming a step is formed inan open end portion of the shaft hole in the housing main body 510 atthe atmospheric air side (A). The first dust seal 100 is fitted in thisannular concave portion 513. Moreover, an annular concave portion 521forming a step is formed in an open end portion of the to-be-fastenedmember 520 at the atmospheric air side (A). The second dust seal 200 isfitted in this annular concave portion 521.

<Dust Seals>

The first dust seal 100 is arranged at the side of the main seal 600,and the second dust seal 200 is arranged at a location nearer to theatmospheric air side than this first dust seal 100. In addition, thefirst dust seal 100 and the second dust seal 200 have no cut portionformed at any location in the circumferential direction thereof.

It is also assumed that the sealing system according to this embodimentis used under a very severe environment such as an iron ore mining site,etc. Accordingly, the dust of which invasion is to be prevented by thedust seals includes, in addition to fine dust such as particulates,those which are apt to adhere to the surface of the shaft 300, such asfrozen substances, mud, grains of iron ore, and so on, and those whichare large and hard to some extent.

The first dust seal 100 has a seal lip which is formed so as to beslidable with respect to the outer peripheral surface of the shaft 300.In addition, the seal lip of this first dust seal 100 is made ofurethane rubber.

Also, the second dust seal 200 has a seal lip which is formed so as tobe slidable with respect to the outer peripheral surface of the shaft300. In addition, the seal lip of this second dust seal 200 is made of arubber (e.g., NBR) which is more flexible than urethane rubber, and iscomposed of only a seal lip extending toward the atmospheric air side.

Here, the seal lip made of urethane rubber is excellent in the functionof scraping dust adhered to the outer peripheral surface of the shaft300. On the other hand, the seal lip made of a rubber, which is moreflexible than urethane rubber, is excellent in the function ofsuppressing the invasion of fine dust.

<Advantages of the Seal System according to this Embodiment>

In the seal system according to this embodiment, the second dust seal200 with its seal lip made of a rubber, which is more flexible thanurethane rubber, is arranged at the atmospheric air side. Accordingly,the invasion of fine dust can be suppressed by means of this second dustseal 200.

In addition, the dust which has invaded from the second dust seal 200and has adhered to the outer peripheral surface of the shaft is scrapedby means of the first dust seal 100 having its seal lip made of urethanerubber. Further, because the seal lip of the second dust seal 200 iscomposed of only a seal lip extending toward the atmospheric air side(A), the dust scraped by the first dust seal 100 is discharged to theatmospheric air side (A) from the side of the seal lip of the seconddust seal 200. Accordingly, it is possible to prevent dust fromaccumulating in a sealed area between the first dust seal 100 and thesecond dust seal 200, thus making it possible to suppress a load frombeing applied to these dust seals.

COMPARATIVE EXAMPLES

Hereinafter, comparative examples in the case of changing theconstruction and materials of seal lips will be explained.

First Comparative Example

In a first comparative example, as a first dust seal, there is adoptedone which is the same as in the above-mentioned embodiment of thepresent invention. In addition, as a second dust seal, there is adoptedone in which the same rubber material as in the embodiment of thepresent invention is used for the material of seal lips thereof, andwhich has a seal lip extending toward a to-be-sealed fluid side (O) inaddition to a seal lip extending toward an atmospheric air side (A).

In this first comparative example, because the seal lip made of arubber, which is higher in flexibility than urethane rubber, is adoptedfor the second dust seal, the invasion of fine dust can be prevented ina suitable manner.

However, the invasion of fine dust can not be completely prevented, sofine dust accumulates in a sealed area between the first dust seal andthe second dust seal with the passage of time. The dust thus accumulatedin this sealed area, being in a state sealed by the first dust seal andthe second dust seal, can not escape from this sealed area. Accordingly,there is a fear that a load may be applied to the first dust seal andthe second dust seal, thus resulting in the damage of the first dustseal and the second dust seal.

Second Comparative Example

In a second comparative example, as a second dust seal, there is adoptedone which is the same as in the above-mentioned embodiment of thepresent invention. In addition, as a first dust seal, there is adoptedone in which a rubber, being higher in flexibility than urethane rubber,is used for the material of seal lips thereof.

In this second comparative example, because the first dust seal and thesecond dust seal are each provided with seal lips made of a rubber,which is higher in flexibility than urethane rubber, the invasion offine dust can be prevented in a suitable manner. However, in such seallips, dust adhered to the outer peripheral surface of the shaft 300 cannot be scraped off, so such dust will invade to the side of the mainseal 600. Accordingly, the sealing ability of the main seal 600 will bespoiled at an early stage of use.

Third Comparative Example

In a third comparative example, as a first dust seal, there is adoptedone which is the same as in the above-mentioned embodiment of thepresent invention. In addition, as a second dust seal, there is adoptedone in which urethane rubber is used for the material of seal lipsthereof.

In this third comparative example, because the first dust seal and thesecond dust seal are each provided with seal lips made of urethanerubber, dust adhered to the outer peripheral surface of the shaft 300can be scraped off in a suitable manner. However, with these seal lips,the invasion of fine dust can not be suppressed. Accordingly, fine dustis present between the seal lip tip end of each seal and the outerperipheral surface of the shaft 300, so each seal lip will be worn outat an early stage of use.

Fourth Comparative Example

In a fourth comparative example, as a first dust seal, there is adoptedone in which a rubber, being higher in flexibility than urethane rubber,is used for the material of seal lips thereof. In addition, as a seconddust seal, there is adopted one in which urethane rubber is used for thematerial of a seal lip thereof.

In this fourth comparative example, because the seal lip of the seconddust seal arranged at an atmospheric air side (A) is made of urethanerubber, the invasion of fine dust can not be suppressed so much by meansof the second dust seal. Accordingly, dust invades into a sealed areabetween the first dust seal and the second dust seal. In addition, theseal lip of the second dust seal is composed of only a seal lipextending toward the atmospheric air side (A), but urethane rubber ishigh in rigidity, so dust can not be caused to discharge to theatmospheric air side (A) in an effective manner, unlike in the case of aseal lip made of a rubber, such as NBR, which is higher in flexibilitythan urethane rubber. Rather, a seal lip of the first dust seal, whichextends toward the atmospheric air side and is made of a rubber, such asNBR, having higher flexibility than urethane rubber, is deformed to ato-be-sealed fluid side (O), thus resulting in the breakage or damage ofthis seal lip.

Next, some more specific examples of the first dust seal 100 and thesecond dust seal 200 will be explained.

First Example

A first dust seal and a second dust seal according to a first example ofthe present invention will be described, while referring to FIG. 2. FIG.2 is a schematic cross sectional view of the first dust seal and thesecond dust seal according to the first example of the presentinvention.

The first dust seal 100 according to this first example is composed of ametal ring 110 having an L-shaped cross section, and a seal body 120that is integrally formed with this metal ring 110. This seal body 120is made of urethane rubber, and is provided with a first seal lip 121that extends to a to-be-sealed fluid side (O), and a second seal lip 122that extends to an atmospheric air side (A). These first seal lip 121and second seal lip 122 are each slidable with respect to a shaft 300.

The first seal lip 121 exhibits a function of suppressing the fluid tobe sealed, which has leaked from a main seal 600, from leaking to theatmospheric air side (A). In addition, the second seal lip 122 exhibitsa function of scraping dust adhered to an outer peripheral surface ofthe shaft 300.

In addition, the second dust seal 200 according to this first example iscomposed of a metal ring 210 having an L-shaped cross section, and aseal body 220 that is integrally formed with this metal ring 210. Thisseal body 220 is made of a rubber (e.g., NBR) which is higher inflexibility than urethane rubber, and is provided with a seal lip 221that extends to the atmospheric air side (A). The second dust seal 200exhibits a function of suppressing the invasion of fine dust. Becausethe seal lip 221 of the second dust seal 200 is constructed to extend tothe atmospheric air side (A), it is caused to bend to the atmosphericair side (A) even by a relatively weak force. Accordingly, dustaccumulated or collected in a sealed area between the first dust seal100 and the second dust seal 200 is discharged to the atmospheric airside (A) from a tip end side of the seal lip 221. As a result, it ispossible to suppress the load of the dust accumulated in this sealedarea from being applied to the first dust seal 100 and the second dustseal 200.

Second Example

A first dust seal and second dust seals according to a second example ofthe present invention will be described, while referring to FIG. 3. FIG.3 is a schematic cross sectional view of the first dust seal and thesecond dust seals according to the second example of the presentinvention.

The first dust seal 100 according to this second example is of the samestructure as in the case of the first dust seal in the above-mentionedfirst example. In this second embodiment, two second dust seals 200 arearranged side by side. Here, note that the structure of each of thesecond dust seals 200 is the same as that of the second dust seal 200 inthe first example.

According to the dust seals of this second example, the invasion of finedust can be suppressed in a more effective manner, as compared with thecase of the first example.

Third Example

First dust seals and a second dust seal according to a third example ofthe present invention will be described, while referring to FIG. 4. FIG.4 is a schematic cross sectional view of the first dust seals and thesecond dust seal according to the third example of the presentinvention.

The second dust seal 200 according to this third example is of the samestructure as in the case of the second dust seal in the above-mentionedfirst example. In addition, in this third embodiment, at an atmosphericair side (A) of a first dust seal 100, which is the same in structure asthat of the above-mentioned first example, there is further arrangedanother first dust seal 100 a in a side by side manner, wherein thefirst dust seal 100 a is constructed so as to be different in structurefrom the first dust seal 100.

This first dust seal 100 a is composed of a metal ring 110 a having anL-shaped cross section, and a seal body 120 a that is integrally formedwith this metal ring 110 a. This seal body 120 a is made of urethanerubber, and is provided with a seal lip 122 a that extends to theatmospheric air side (A). This first dust seal 100 a is different fromthe first dust seal 100 in that it is not provided with a seal lip whichextends to a to-be-sealed fluid side (O).

The seal lip 122 a of the first dust seal 100 a exhibits a function ofscraping dust adhered to an outer peripheral surface of a shaft.

According to the dust seals of this third embodiment, the dust adheredto the outer peripheral surface of the shaft can be scraped off in amore efficient manner, as compared with the case of the first example.Here, note that the first dust seal 100 a is not provided with the seallip which extends to the to-be-sealed fluid side (O), so the dustscraped by the first dust seal 100 is discharged to the atmospheric airside (A) from a tip end side of the seal lip 122 a of the first dustseal 100 a.

Fourth Example

A first dust seal and second dust seals according to a fourth example ofthe present invention will be described, while referring to FIG. 5. FIG.5 is a schematic cross sectional view of the first dust seal and thesecond dust seals according to the fourth example of the presentinvention.

The first dust seal 100 according to this fourth example is of the samestructure as in the case of the first dust seal in the above-mentionedfirst example. In this fourth embodiment, three second dust seals 200are arranged side by side. Here, note that the structure of each of thesecond dust seals 200 is the same as that of the second dust seal 200 inthe first example.

According to the dust seals of this fourth example, the invasion of finedust can be suppressed in a more effective manner, as compared with thecase of the second example.

Fifth Example

First dust seals and second dust seals according to a fifth example ofthe present invention will be described, while referring to FIG. 6. FIG.6 is a schematic cross sectional view of the first dust seals and thesecond dust seals according to the fifth example of the presentinvention.

In this fifth embodiment, the same structure as in the above-mentionedthird example is adopted with respect to the first dust seals. That is,a first dust seal 100 provided with a first seal lip 121 and a secondseal lip 122 and another first dust seal 100 a provided only with a seallip 122 a extending to an atmospheric air side (A) are arranged side byside.

In addition, the same structure as in the above-mentioned second exampleis adopted with respect to the second dust seals. That is, two seconddust seals of the same structure as that of the second dust seal 200 ofthe first example are arranged side by side.

According to the dust seals of this fifth example, the invasion of finedust can be suppressed in a more effective manner, and dust adhered toan outer peripheral surface of a shaft can be scraped off in a moreefficient manner, as compared with the case of the first example.

Sixth Example

A first dust seal and a second dust seal according to a sixth example ofthe present invention will be described, while referring to FIG. 7. FIG.7 is a schematic cross sectional view of the first dust seal and thesecond dust seal according to the sixth example of the presentinvention.

The second dust seal 200 according to this sixth example is of the samestructure as in the case of the second dust seal in the above-mentionedfirst example. In addition, as the first dust seal 100 a according tothis sixth embodiment, there is adopted one which is provided only witha seal lip 122 a extending to an atmospheric air side (A), as shown inthe above-mentioned third and fifth examples.

According to the dust seals according to this sixth embodiment, the sameeffects as in the case of the above-mentioned first example can beobtained except that the function of suppressing the fluid to be sealed,which has leaked from the main seal 600, from leaking to the atmosphericair side (A) is not provided.

(Others)

In the above-mentioned embodiment, there has been shown a constructionin which the first dust seal 100 is fitted in the annular concaveportion 513 formed in the housing main body 510, and the second dustseal 200 is fitted in the annular concave portion 521 formed in theto-be-fastened member 520.

However, in cases where two or more first dust seals 100 and two or moresecond dust seals 200 are used, there is not necessarily a need to fitall the first dust seals 100 in the annular concave portion 513, and tofit all the second dust seals 200 in the annular concave portion 521. Inother words, as long as a first dust seal 100, which is arranged at alocation nearest to the to-be-sealed fluid side (O), is fitted in theannular concave portion 513, and a second dust seal 200, which isarranged at a location nearest to the atmospheric air side (A), isfitted in the annular concave portion 521, the other dust seals may befitted in either of the annular concave portions.

While the invention has been described in terms of the preferredembodiment and some examples, those skilled in the art will recognizethat the invention can be practiced with modifications within the spiritand scope of the appended claims.

1. A sealing system which serves to seal an annular gap between an innerperipheral surface of a shaft hole formed in a housing and an outerperipheral surface of a shaft which is inserted through said shaft holeand carries out a reciprocating movement relative to said housing, saidsystem comprising: a main seal that prevents fluid to be sealed fromleaking to an atmospheric air side; and a dust seal that is arranged ata location nearer to the atmospheric air side than said main seal, forpreventing dust from invading to a side of the fluid to be sealed;wherein said dust seal is composed of a first dust seal and a seconddust seal which is arranged at a location nearer to the atmospheric airside than said first dust seal, and each of said first dust seal andsaid second dust seal has a seal lip formed so as to be slidable withrespect to the outer peripheral surface of said shaft; the seal lip ofsaid first dust seal is made of urethane rubber; and the seal lip ofsaid second dust seal is made of a rubber which is higher in flexibilitythan said urethane rubber, and is composed of only a seal lip thatextents toward the atmospheric air side.